Pollution control devices are employed on motor vehicles to control atmospheric pollution. Such devices include a pollution control element. Exemplary pollution control devices include catalytic converters and diesel particulate filters or traps. Catalytic converters typically contain a ceramic monolithic structure having walls that support the catalyst. The catalyst typically oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in engine exhaust gases to control atmospheric pollution. The monolithic structure may also be made of metal. Diesel particulate filters or traps typically include wall flow filters that are often honeycombed monolithic structures made, for example, from porous ceramic materials. The filters typically remove soot and other exhaust particulate from the engine exhaust gases. Each of these devices has a housing (typically made of a metal like stainless steel) that holds the pollution control element.
Monolithic pollution control elements, are often described by their wall thickness and the number of openings or cells per square inch (cpsi). In the early 1970s, ceramic monolithic pollution control elements with a wall thickness of 12 mils and a cell density of 300 cpsi were common (“12/300 monoliths”). As emission laws become more stringent, wall thicknesses have decreased as a way of increasing geometric surface area, decreasing heat capacity and decreasing pressure drop of the monolith. The standard has progressed to 6/400 monoliths.
With their thin walls, ceramic monolithic structures are fragile and susceptible to vibration or shock damage and breakage. The damaging forces may come from rough handling or dropping during the assembly of the pollution control device, from engine vibration or from travel over rough roads. The ceramic monoliths are also subject to damage due to high thermal shock, such as from contact with road spray.
The ceramic monoliths have a coefficient of thermal expansion generally an order of magnitude less than the metal housing which contains them. For instance, the gap between the peripheral wall of the metal housing and the monolith may start at about 4 mm, and may increase a total of about 0.33 mm as the engine heats the catalytic converter monolithic element from 25° C. to a maximum operating temperature of about 900° C. At the same time, the metallic housing increases from a temperature of about 25° C. to about 530° C. Even though the metallic housing undergoes a smaller temperature change, the higher coefficient of thermal expansion of the metallic housing causes the housing to expand to a larger peripheral size faster than the expansion of the monolithic element. Such thermal cycling typically occurs hundreds or thousands of times during the life of the vehicle.
To avoid damage to the ceramic monoliths from road shock and vibrations, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monoliths and the metal housings (thereby bypassing the catalyst), mounting mats or mounting paste materials are disposed between the ceramic monoliths and the metal housings. The process of placing the monolith within the housing is also called canning and includes such steps as wrapping a sheet of mat material around the monolith, inserting the wrapped monolith into the housing, pressing the housing closed, and welding flanges along the lateral edges of the housing. The paste may be injected into the gap between the monolith and the metal housing, perhaps as a step in the canning process.
Typically, the paste or sheet mounting materials include inorganic binders, inorganic fibers, intumescent materials, organic binders, fillers and other adjuvants. The materials may be used as sheets, mats, or pastes. Known mat materials, pastes, and intumescent sheet materials used for mounting a monolith in a housing are described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.), U.S. Pat. No. 4,305,992 (Langer et al.), U.S. Pat. No. 4,385,135 (Langer et al.), U.S. Pat. No. 5,254,410 (Langer et al.), U.S. Pat. No. 5,242,871 (Hashimoto et al.), U.S. Pat. No. 3,001,571 (Hatch), U.S. Pat. No. 5,385,873 (MacNeil), U.S. Pat. No. 5,207,989 (MacNeil), GB 1,522,646 (Wood), Japanese Kokai No.: J.P. Sho. 58-13683 (i.e., Pat Appln Publn No. J.P. Hei. 2-43786 and Appln No. J.P. Sho. 56-112413), and Japanese Kokai No.: J.P. Sho. 56-85012 (i.e., Pat. Appln No. Sho. 54-168541). Mounting materials should remain very resilient at a full range of operating temperatures over a prolonged period of use.
To continually improve emission standards, it has been desired to move catalytic converters closer to the engine and thereby increase the temperature of the exhaust gasses traveling through the catalytic converter. The hotter catalytic converter and exhaust gasses therein increase the efficiency of the reactions, which remove pollution from the exhaust gasses. As hotter catalytic converter temperatures are used, the mounting materials must be able to withstand the severe temperatures. In addition, the thermal transmission properties of the mounting material become more important toward protecting closely mounted engine components from the hot exhaust temperatures. Decreasing the converter skin temperature is important in preventing heat damage in the engine compartment and radiation into the passenger compartment.
It has also been desired to continually decrease wall thicknesses of the ceramic monolithic structure to enhance catalytic converter operation. Extremely thin wall monoliths, such as 4/400, 4/600, 3/600, 3/900, 2/900 monoliths, and 2/1200 have been developed or are expected to be developed in the not too distant future. The monoliths with extremely thin walls are even more delicate and susceptible to breakage. Typical intumescent mounting structures provide compression pressures which increase during use of the catalytic converter to a pressure above the initial mounting pressure. Increasing compression pressures during use of the catalytic converter also reduce the ability of support mats or pastes to sufficiently insulate the monolith from vibration damage or mechanical shock. Because of these various problems, published reports have advised against using intumescent mounting mats for extremely thin wall monoliths mounted close to the engine. See for example Umehara et al., “Design Development of High Temperature Manifold Converter Using Thin Wall Ceramic Substrate”, SAE paper no. 971030, pg. 123-129, 1997.
A need exists for a mounting system which is sufficiently resilient and compressible to accommodate the changing gap between the monolith and the metal housing over a wide range of operating temperatures and a large number of thermal cycles. While the state of the art mounting materials have their own utilities and advantages, there remains an ongoing need to improve mounting materials for use in pollution control devices. Additionally, one of the primary concerns in forming the mounting mat is balancing between the cost of the materials and performance attributes. It is desirable to provide such a high quality mounting system at the lowest possible cost. Because of increasing environmental concerns, the mounting mat is preferably also more environmentally friendly.